As is known, in many optical modules in which an optical emitter, such as a laser or an end of an optical fibre from which light is emitted, is in optical communication with another part of an optical communication system along a main optical path, one or more other optical components are arranged to monitor or measure various characteristics of the light in the main optical path. For example, one or more beam splitters are often arranged to direct a small portion of the light from the main optical path to one or more photodiodes to enable the power of the light to be measured. Sometimes the light directed out of the main optical path is passed through one or more other optical components, such as a wavelength dependent filter, to enable the power measured by the photodiodes to be used to determine wavelength as part of a wavelength locker, in which, for instance, the wavelength of a laser is controlled by controlling the temperature of the laser, or for other purposes.
Such optical modules, often, therefore include several photodiodes arranged at different positions in the optoelectronic assembly and electrical signals from the photodiodes are routed through and out of the assembly to a controller which responds to the signals. The photodiodes are usually mounted on ceramic blocks having conductive tracks thereon to which the photodiode is electrically connected using die and wire bonds. The conductive tracks are continuous over at least two sides of the ceramic blocks and further wire bonds are made between the conductive tracks on the ceramic blocks and further conductive tracks provided on a substrate on which the ceramic blocks and the various optical components are mounted. The conductive tracks on the substrate lead to an edge of the substrate from where further wire bonds are provided to a further substrate outside the optical module, which may be hermetically sealed. These further wire bonds between the edges of the two substrates may disadvantageously conduct heat between the two substrates, thereby perturbing the control of the wavelength of the laser, for example.
As is apparent, therefore, such an optical module includes a number of photodiodes on ceramic blocks, which are relatively expensive components, having conductive tracks extending over at least one edge of the block, which is difficult to manufacture, and having a number of wire bonds, which need to be individually made. Furthermore, the photodiodes are distributed over the substrate, thereby requiring individual placement and alignment, and they may operate at different ambient temperatures, thereby ageing at different rates and producing signals which may have different temperature dependencies.